Entropy without a ceiling

A cosmological model in which the dark sector is the zero-energy residual of gravitational entropy. A single vacuum term, the model's dark energy, follows Penrose's gravitational-entropy indicator, the ratio of Weyl to Ricci curvature, used as a timing signal and called the clock. The ratio starts at zero in a smooth early universe, rises as structure forms, peaks near z ≈ 0.9 and then falls. While it rises, dark matter gives up energy to the vacuum; while it falls, the vacuum releases energy to a stiff helicity-0 mode. Global zero energy (E = 0), read through unimodular gravity, fixes how much vacuum energy exists, and gravitational entropy fixes when it changes. Fit to DESI DR2 baryon acoustic oscillations, DES-SN5YR supernovae and the Planck acoustic scale, the model reaches Δχ² ≈ −28.4 relative to ΛCDM. That matches the two-parameter w0wa fit with no free parameter beyond ΛCDM's h. The clock's coarse-graining mass is set in advance by the galaxy cooling limit (1.26 × 10¹² solar masses), and ε = 1 follows from a boundary condition. The linear, constant-coefficient coupling is derived. The squared alternative exhausts the dark matter, and an H²-weighted form fails by more than 7 × 10⁴ in χ². Twelve physically motivated alternatives fit the same data worse. The helicity-0 mode of a massive graviton (m_g ~ H0) supplies the required exchange: no measurable lag; a loss tangent ≲ 5 × 10⁻⁴; homogeneity through Vainshtein screening; the stiff sink (kination). Its mass also accounts for the constant coefficient, because the suppression factor (m_g/H)² cancels the H² weighting. Two points remain open: the microscopic origin of the linear response, and a tension at the crossover where H falls to m_g. Predictions: a dark-sector density peak at z ≈ 0.9, with the inferred w still crossing −1 near z ≈ 0.5; about 7% dark-matter conversion; a late released component of ~9% of the critical density; ~5% excess growth (S8 ≈ 0.85), the model's weakest point; a locked relation between w(z) and fσ8(z). The vacuum fades as a power law, so the horizon opens and total entropy rises without bound, with no de Sitter ceiling. DESI DR3, Euclid and Rubin can test this now. Claude (Anthropic) was used as an analytical and writing tool and is not credited as an author.